1. Field of the Invention
The present invention relates to a light-emitting element of a novel structure employing semiconducting diamond films. It is capable of emitting a short wavelength light, such as bluish light.
2. Description of the Prior Art
Diamond has a wide band gap of 5.5 eV and is expected to be used for solid-state devices for emitting light in a short-wavelength region. P-type semiconducting diamond films doped with boron (B) can be produced by vapor-phase synthesis. Research and development activities have been conducted to develop semiconducting diamond electroluminescence (EL) devices, capable of emitting a short wavelength light, employing the p-type semiconducting diamond films.
Such a semiconducting diamond EL element having a structure as shown in FIG. 5 is proposed in Japan Applied Physics Society 1989 Spring Meeting Abstract, Vol. 2, P. 481. Referring to FIG. 5, the semiconducting diamond EL element comprises a single crystal diamond substrate 51, a p-type semiconducting diamond layer 52 formed over the surface of the diamond substrate 51 by vapor-phase synthesis and doped with B, a Schottky electrode 53 formed of tungsten (W) in Schottky contact with the p-type semiconducting diamond layer 52, and an ohmic electrode 54 formed of titanium (Ti) in ohmic contact with the p-type semiconducting diamond layer 52.
Shown in FIG. 6 is the energy band diagram for the semiconducting diamond EL element of FIG. 5 when a voltage V is applied across the Schottky electrode 53 (positive) and the ohmic electrode 54 (negative). In FIG. 6, indicated at E.sub.F is the Fermi level, indicate at E.sub.C is the energy at the lower end of the conduction band of the p-type semiconducting diamond layer 52, and indicate at E.sub.V is the energy at the upper end of the valence band of the p-type semiconducting diamond layer 52. In FIG. 6, holes are represented by open circles, and electrons are represented by solid circles. Holes can move by a quantum effect from the Schottky electrode 53 to the p-type semiconducting diamond layer 52, and then recombine with electrons at the defect levels to emit a bluish light.
Since this EL element uses a rectifying Schottky contact on the p-type semiconducting diamond layer, the Schottky characteristic is sensitively dependent on both the boron doping concentration and the surface condition of the p-type semiconducting diamond layer. Therefore, it is difficult to form the Schottky electrode with a predetermined Schottky characteristic. Furthermore, since the wavelength of the light is greatly dependent on the doping concentration in the p-type semiconducting diamond layer, which is a light emitting layer, it is difficult to change the degree of doping without changing the predetermined Schottky characteristic, and hence it is also difficult to determine the color of light to be emitted. Finally, since the reverse bias voltage applied to the EL element should be limited in a narrow voltage range slightly below the breakdown voltage, it is difficult to control the voltage to vary the EL intensity.